1. Field of the Invention
The present invention relates to a signal processing apparatus.
2. Description of the Related Art
As represented by a digital VCR (Video Cassette Recorder), a data recording and reproducing apparatus that records a digital video signal and a digital audio signal to a record medium and that reproduces them therefrom is known. Since the data capacity of a digital video signal is huge, it is normally compressed and encoded before it is recorded on the record medium. In recent years, the MPEG2 (Moving Picture Experts Group 2) system is known as a standard compressing and encoding system.
In picture compression technologies including the foregoing MPEG2 system, the data compression rate is improved using a variable length code. Thus, depending on the complexity of a picture to be compressed, the code amount of one screen (for example, one frame or one field) that has been compressed varies.
The forgoing MPEG2 system has a hierarchical structure composed of a macro block layer, a slice layer, a picture layer, a GOP layer, and a sequence layer disposed in the order from the bottom to the top. In the hierarchical structure, the slice layer is a unit of a variable length code encoding process. The macro block layer is composed of a plurality of DCT (Discrete Cosine Transform) blocks. At the beginning of each layer, a header portion that stores header information is disposed. For example, in the slice layer, by detecting the header portion, the delimitation position of a variable length code is detected. A decoder decodes a variable length code corresponding to the detected delimitation positions thereof.
When an apparatus does not output a significant signal (for example, when a video camera is in standby state, when a video monitor apparatus is in no signal input state, when a video recorder is in stop state or standby state, or when a switcher device is in no signal input state), it is not suitable to output a video signal of a noise contaminated picture. In addition, when a video signal of a noise contaminated picture is input, the synchronization between such an apparatus and another apparatus connected thereto is lost. Such a situation is not also permitted. To prevent such situations, conventionally, apparatuses that are used in broadcasting stations and professional use customarily output a black picture (a video signal that causes a black screen to be displayed). Such apparatuses may output a blue picture, a green picture, or a pink picture.
In the conventional apparatuses, when they do not output a significant signal, they output a fixed video signal. In the conventional video apparatuses, base band signals that have not been compressed are input, internally processed, and output. Signals that are handled in the apparatuses are only video signals composed of G, B, and R or Y, Cb, and Cr (that are a luminance signal and color difference signals). Thus, on any stage before a video signal is output from a video apparatus, when the video signal is clamped with G=0, B=0, and R=0 or Y=16, Cb=128, and Cr=128 (in the case of an eight-bit digital system), the subsequent video signal is treated as a black picture. As a result, the video apparatus outputs a black picture.
Such an operation applies to the forgoing video camera, video monitor apparatus, video recorder, switcher apparatus, and so forth. This theory also applied to a digital video apparatus that handles a non-compressed digital video signal, an analog video apparatus that handles an analog video signal, and a composite video apparatus that handles a composite video signal.
On the other hand, in the forgoing MPEG2 stream, although value 0 represents no signal (stuffing), it does not represent a black picture. For example, when no signal is input, an EE signal (monitor signal) becomes non-signal state. Likewise, while a tape stops, a reproduction signal becomes non-signal state. Thus, such signals are not decoded as black signals.
However, in recent years, a recording apparatus that inputs a video signal as a non-compressed base band signal, compresses and encodes the video signal with a variable length code corresponding to the MPEG2 system or JPEG (Joint Photographic Experts Group) system, and records the resultant signal to the record medium has been released. In addition, a recording and reproducing apparatus that directly inputs and outputs a data stream that has been compressed and encoded with a variable length code and records and reproduces the data stream has been proposed. In such a recording and reproducing apparatus, a data stream that has been compressed and encoded corresponding to for example the MPEG2 system is directly input thereto and directly output therefrom.